Coherent polarizations in the band-to-band continuum of bulk GaAs are studied in pump-probe experiments with 20 fs pulses. For the first time, we observe quantum beats on a 100 fs time scale that are due to an impulsively excited quantum coherence between heavy and light hole states. The beat frequency is determined by the heavy-light hole energy splitting changing continuously with the energy separation between the laser and the band gap. Theoretical calculations of the coherent response based on the semiconductor Bloch equations for a three-band scheme account for the data.[S0031-9007(96)02256-9] PACS numbers: 78.47.+ p, 42.65.Re, 78.20.Ci The fundamental nonequilibrium dynamics of optically excited semiconductors occurs on ultrafast time scales. Spectroscopy with femtosecond laser pulses provides direct information on such phenomena and has identified a coherent regime of material response in which the nonlinear polarization of the material and the electric field of the pulse couple in a phase-coherent way. In semiconductors with a direct band gap, both excitonic and free carrier excitations give rise to coherent polarizations with distinctly different properties [1-9]. Excitonic polarizations have been studied in great detail and new phenomena like wave packet propagation [1] and/or beating phenomena [2-4] have been observed in femtosecond four-wavemixing experiments where the pulse spectrum overlaps with transitions of different frequency. For an ensemble with different transition frequencies, e.g., heavy hole excitons of different binding energy in quasi-two-dimensional (2D) structures, the oscillatory overall polarization originates from the contributions from the individual two-level constituents [2,4]. A different situation exists if the transitions are coupled via a common state and quantum interference in three-level systems causes polarization beats for heavy hole (HH) and light hole (LH) excitons [3][4][5]. In 2D systems, studies of the coherent response are facilitated by the-relatively slow-picosecond phase relaxation of excitonic polarizations.Much less is known on nonlinear polarizations in the band-to-band continuum of semiconductors, showing dephasing kinetics in the sub-100-fs regime [6]. Recent four-wave-mixing experiments using sub-20-fs pulses close to the band gap of bulk GaAs [7] and bulk CdSSe [8] gave evidence of oscillatory coherent polarizations. In Ref.[7], the oscillations were attributed to the coherent coupling of interband transitions and LO (longitudinal optical) phonons, giving insight into nonMarkovian quantum kinetics. The much higher oscillation frequency in CdSSe was ascribed to intervalence band quantum beats of free carriers. In such experiments, however, the pulse spectrum overlapped with the absorption edge of the material where (i) excitonic effects are important, and (ii) the strong variation of absorption across the pulse spectrum leads to nonlinear propagation effects [9] and/or detuning oscillations [10]. Thus, optical excitation well above the absorption edge i...

The nonlinear reflectivity of semiconductor saturable absorber mirrors has been investigated as a function of incident energy fluence. The presence of two-photon absorption in commonly used structures was confirmed via time-resolved differential reflectivity measurements. Theoretical calculations predict that two-photon absorption will expand the continuous-wave mode-locking stability regime against Q-switched mode-locking, yet may simultaneously induce multiple pulses in a laser cavity.

Femtosecond mid-infrared laser pulses that are continuously tunable in the wavelength range from 9 to 18mum are demonstrated. These nearly bandwidth-limited pulses are generated by phase-matched difference-frequency mixing within the broad spectrum of 20-fs pulses from a mode-locked Ti:sapphire laser in GaSe. A direct determination of the pulse duration at 11.5mum gives a value of 140 fs. The average mid-infrared power of 1muW is ~100 times greater than that for infrared generation by non-phase-matched optical rectification.

We present a detailed and systematic investigation of carrier capture, relaxation, cooling and radiative recombination in a one-dimensional semiconductor quantum wire of high structural perfection and optical quality over a large range of excitation (carrier) densities. Experimental evidence for a complete lack of 1D bandgap renormalization is found. Even u p to high carrier densities, > 106cm-', where strong band filling is already present and directly visible in the luminescence, no shift of bandgap to low energy is found. The carrier cooling in I D is appreciably slower than in comparable 2D structures, t h u s leading to high carrier temperatures. This confirms theoretical predictions of reduced phonon scattering probability in one-dimensional structures. The temoerature deoendence of the radiative lifetime of the 1D carriers is investigated. The theoretically predicted ,,6 dependence is not found. On the contrary an empirical = 0.02 T n s K -' law is fulfilled.

The ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors are investigated using reflective pump–probe measurements. At high fluence, ultrafast induced absorption begins to dominate over absorption bleaching. Above the InGaAs quantum well band gap, the differential reflectivity shows a ∼1 ps transient due to nonequilibrium carrier dynamics. Below band gap, the signal is dominated by a strong two-photon absorption component followed by induced absorption that decays with a time constant of ∼5 ps; these components are attributed to nonlinear absorption and subsequent carrier diffusion in the InP layer.